1. Field of the Invention
The present invention relates to electromagnetic actuators, and more specifically, to an electromagnetic actuator of which a housing of the movable core constitutes part of the magnetic circuit.
2. Description of the Related Art
As disclosed in Japanese Patent Laid-Open Publication No. 2001-332419, a known conventional electromagnetic actuator is equipped with a housing for holding a movable core so that it may freely reciprocate back and forth and a stator having an attraction part that exerts a magnetic attractive force on the movable core in either of the reciprocating directions. The stator is configured together with the movable core to form a magnetic circuit of magnetic flux produced by running electric current in the coil.
In the above type electromagnetic actuator however, the housing and the movable core slide directly in contact with each other, and therefore the wear of their sliding faces is a problem.
The inventors have found that Ni—P plating or Ni—P plating plus heat treatment on the sliding face of the movable core and gas soft nitriding of the sliding face of the housing, both for improving wear-resistance of the sliding faces, causes problems. Such an electromagnetic actuator equipped with a linear electromagnetic valve mechanism having the above surface-treated sliding faces may be employed in a hydraulic control valve that controls the hydraulic pressure of operation oil supplied to the hydraulic pressure control device of an automatic transmission of a vehicle. Then, although the operation oil pressure controlled by a coil current is within a demanding tolerance, the position of the movable core determined by the same coil current varies depending on the moving direction of the movable core. Additionally, a relatively large hysteresis (attractive force hysteresis) is observed.
As a result of an intensive study on the causes for such hysteresis, the inventors have discovered that a 1–2 μm thick porous layer is formed in the surface of the gas soft nitrided sliding face and that this porous layer causes the relatively large hysteresis.
In addition, if the electromagnetic actuator is used for a long time, the porous layer peels off, and sliding problems arise. In the electromagnetic valve disclosed in Japanese Patent Laid-Open Publication No. Hei. 4-221810, the movable ferrite core is nitrided (by tufftride treatment) to harden its surface and its surface roughness is raised by wrapping, in order to reduce friction with the guide material. Removal of the porous layer at random, however, will lower productivity. Through further investigation into this problem, the inventors have discovered that the amount of wear decreases significantly if surface roughness is 3.2 Rz or lower, as shown in FIG. 5, which describes the relationships between surface roughness and the amount of wear.